The present invention relates to intrusion detection systems. More particularly, it relates to a system that differentiates between a vehicle intrusion event and a non-intrusion event.
Intrusion detection systems are known for protection of vehicles and their contents from theft. If an intrusion is detected, known systems sound the vehicle""s horn, flash the vehicle""s lights, and/or disable the vehicle""s ignition system to render the vehicle inoperative.
Intrusion detection systems utilizing ultrasonic signals and the Doppler principle are also known. Such devices detect a change in the received frequency from the transmitted signal frequency, the difference frequency being a Doppler frequency resulting from a Doppler shift due to movement within a monitored area. The received signal is a sinusoidal waveform having a frequency dependent upon the rate of change in position of an object within the protected area. When there is no motion within the protected area, the frequency of the return signal is equal to the ultrasonic frequency of the transmitted signal, i.e., there is no Doppler shift when there is no object movement.
Intrusion detection systems using ultrasonic signals, however, are prone to false alarms. Ideally, the system should not detect non-intrusive events that may occur relative to the vehicle. An inadvertent striking of the outside of one of the vehicle""s windows, motion near or around the vehicle, or air turbulence within the occupant seating area are non-intrusive events that should not trigger the vehicle""s intrusion detection system. Nevertheless, these non-intrusive events alter the signal received by an ultrasonic intrusion detection systems by modulating or adding noise to the reverberation field generated by the transmitter and received by the receiver, i.e., the non-intrusive event may be interpreted as being an intrusive event which, in turn, results in a false alarm.
It is known in intrusion detection ultrasonic-based systems to decrease the system""s sensitivity to reduce the possibility of a false alarm. The detection thresholds may be raised to decrease the system""s sensitivity to non-intrusive events which will reduce the occurrence of false alarms, but generally at the expense of reduced intrusion detection capability. Further, temperature changes in the vehicle""s interior can disturb the amplitude and/or phase of the return signal received by the intrusion detection system. Thus, it is desirable for the intrusion detection system to not interpret a temperature disturbance as an intrusion event.
One particular type of known intrusion detection system generates a reverberation field within a protected space. The reverberation field includes a plurality of signals traveling along a plurality of propagation paths within the protected space. The system detects a change in the characteristic of the reverberation field upon the changing of a physical characteristic of an object within the protected space. For instance, the entry of a new object into the field or a change in position of motion of an existing object in the field. An alarm signal is only generated when the change in the reverberation field is greater than a predetermined threshold value.
Another type of known intrusion detection system detects intrusion into a vehicle interior by passing an airborne acoustic signal through a portion of the vehicle interior when the vehicle security system is active. The system measures any change in phase in the signal as it passes through the vehicle. The phase of a first frequency component in the signal is sensitive to intrusion into the vehicle. The phase of a second frequency component is sensitive to false alarm conditions, and is not sensitive to intrusion.
In accordance with the present invention, an apparatus and method are provided for detecting intrusion or non-intrusion events within limits of a predetermined area. In accordance with one aspect, the intrusion/non-intrusion detection system transmits and receives continuous wave signals within the limits of the predetermined area. The system includes signal processing means for generating envelope waveforms from the returns of the continuous wave signal. Further, the system includes a controller that divides the envelope waveforms into windows and compares the envelope waveforms with predetermined thresholds to determine the occurrence of an intrusion or non-intrusion event. For instance, the entry of a new object into the field or a change in position of motion of an existing object in the field.
In accordance with one aspect of the present invention, an apparatus includes a transmitter transmitting a continuous wave signal within the predetermined area, a receiver receiving reflected return signals from the continuous wave signal, a demodulator demodulating the return signals, monitor monitoring envelopes of the demodulated return signals during time windows, and a discriminator discriminating between an intrusion and a non-intrusion event in response to the monitored envelopes during the time windows.
In accordance with another aspect of the present invention, a method for detecting intrusion within limits of a predetermined area includes the steps of transmitting a continuous wave signal within the predetermined area, receiving returns of the continuous wave signal bounced within the limits of the predetermined area, demodulating the return signal, monitoring envelopes of the demodulated return signals during time windows, and discriminating between an intrusion and a non-intrusion event in response to the monitored envelopes during the time windows.